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The critical voltage effect in transmission electron microscopy. II. A theoretical study neglecting absorption effects
Author(s) -
Gevers R.,
Serneels R.,
David M.
Publication year - 1975
Publication title -
physica status solidi (b)
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.51
H-Index - 109
eISSN - 1521-3951
pISSN - 0370-1972
DOI - 10.1002/pssb.2220670126
Subject(s) - condensed matter physics , degenerate energy levels , bloch wave , voltage , physics , optics , acceleration voltage , antisymmetric relation , symmetry (geometry) , electron , molecular physics , chemistry , quantum mechanics , geometry , mathematics , cathode ray , mathematical physics
Using degenerate perturbation theory a study is made of the critical voltage effect in electron microscopy. In particular the dependence on the accelerating electron voltage of the profile of the second order bend contour for centrosymmetric crystals is considered. The case of non‐centrosymmetric crystals is included in the formulae. It is found for centro‐symmetrical crystals that the more the critical voltage is approached, the second order bend contour appears as a kinematical like bend contour which disappears completely in a small interval around the critical voltage. Also the symmetry properties are discussed of the Bloch waves corresponding to the branches of the dispersion surface which touch at the critical voltage. It is shown that away from the critical voltage and when the second order reflection is not exactly satisfied the Bloch waves are nearly symmetric or antisymmetric. When the voltage is varied towards the critical voltage a continuous change appears of the Bloch wave symmetry in a given interval around the critical voltage. The smaller the mis‐orientation the smaller is the voltage interval. For non‐centrosymmetric crystals a similar effect is observed.